How to scale the rendered image in composition?
Yes, Blender can scale in the compositor using the Scale nodes seen below. Pipe the image into a Scale node, then from the Scale node into a Composite output node. If you have "Compositing" checked on in the Rendering tab of the Properties pane, then each frame will be modified and then saved out as usual. 



You can also use a File Output node to save out different files in addition to the final Composite, if you want.

Note that in Absolute Mode for the Scale node, you need to input the final image size you're after, not a ratio, so if you want a full HD, you should input 1920 for X, 1080 for Y.